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BLM Technical Note 380 








'^- — *•* 




As the Nation's principal conservation agency, the Department of 
the Interior has basic responsibilities for water, fish, wildlife, mineral, 
land, park, and recreational resources. Indian and Territorial affairs are 
other major concerns of this department of natural resources. 

The Department works to assure the wisest choice in managing 
all our resources so that each shall make its full contribution to a better 
United States now and in the future. 

£fl3 XO'^^^HP^^ 

BLM Technical Note 380 



Wallace A. Crisco 


BLM Service Center 

August 1988 


Available from: Printed Materials Distribution Section 
BLM Service Center (D-558B) 
Denver, CO 80225-0047 


National Technical Information Service 
Springfield, VA 22161 

U.S. Department of the Interior 

Bureau of Land Management <l^ 


This publication is designed to provide the necessary content for a basic 
aerial photo interpretation course. The order in which subjects are addressed 
may need to be varied. The depth of instruction about various portions may 
need modification depending on the experience or training background of the 
audience. It is hoped that the utilization of this material in conjunction 
with site specific information (field site photo's and other data) will 
provide the basic components (off the shelf) for many Photo Interpretation 
Training courses. 

<1 ^Zs^^ 



Page No, 

1. Introduction L 

2. Elements of Aerial Photography 

o Types of Photography 2-3 

o Fi 1ms 4 

o Filters 4 

o Photographs 5 

o Photography Acquisition 6-7 

o Titling and Indexing 8-9 

o Displacements and Distortions 10-13 

o Scale Considerations 14-17 

3. Photo Interpretation and Usage 

o Scale Determinations 18-19 

o Acreage Determination 20-21 

o Image Characteristics 22-31 

o Techniques and Procedures 32 

o Photo Preparation 33-35 

o Vegetation Overlay 37 

o Drainage Overlay 39 

4. Acknowledgements 



This publication deals with aerial photographs and how they can be used in 
the various phases of land management within the Bureau of Land Management. 
It is intended to furnish sufficient guidelines to encourage the use of aerial 
photos. Emphasis has been placed on basic data in this publication for its 
general application to Bureau activities where aerial photos can be 
appropriately used. 

The aerial photograph offers the easiest, most complete way to examine an 
area in it's entirety. It's unique portrayal of all visible detail makes it 
possible to analyze, measure, quantify and interpret more information than can 
be obtained from any other single source. It has revolutionized topographic 
mapping and brought new search techniques to such diverse fields as geology 
and archaeology. 

An aerial photograph is simply a photograph of the surface of the earth. 
It may approximate the normal view from an elevation but more likely it will 
have the unfamiliar view of the scene directly below the airplane. The Bureau 
can make classifications of land with a high degree of accuracy by skilled 
photo interpreters. Soils, building materials, and timber stands are being 
accurately determined in the same manner. The increased workload and 
decreased workforce necessitates advanced planning and programming to utilize 
photo interpretation to collect and analyze reliable data for managing the 
public lands. 

Even the fact that a photograph grows old is of value. The record made by 
a photograph is so inclusive at tne time it is taken that it becomes a 
valuable historical document and will serve as an excellent reference for 
measuring our progress and accomplishments on the land itself. 

Recognition of the value of the aerial photograph comes from use and 
understanding of the photograph's limitations as well as its capabilities. If 
fully utilized, aerial photographs are a tool to make our work easier and 
enables us to do it better. This publication is aimed at presentation of 
fundamentals in the use of aerial photography. 

Vertical photography 



i<~ — " 

Most commonly used 






Low oblique photography 



(Horizon is not included) 

Less commonly used 

/•I 1 

V \ 

/ / 

\ \ 

/ / 

\ \ 

/ / 

\ \ 

' / 


hi 1 



High oblique photography 

r \ ,. , 5 /-' RELATION OF 


r\^'W^r^% THE GROUND. 

^*-^ ^i&SWt&fr. ^ 

(Horizon Is included) 

Reconnaissance photography 










Figure I. Types of aerial photography 


Types of photography 

Three common types of aerial photography are: Vertical, Low oblique, and 
High oblique (Figure 1). 

The vertical photograph is taken from a viewpoint directly below the 
airplane. This gives a map-like photograph of the earth's surface, however, 
it is not a map. The map-like qualities are emphasized only where the terrain 
is flat and level. A grid in that case will be relatively square and suitable 
planimetric maps can be prepared directly by overlays because corrections are 
easy. Distortions and displacements will require some corrections. 

This is the most commonly used photograph. All parts of the terrain are 
visible, scale is fairly uniform throughout, except in mountainous terrain, 
and complex methods are not needed to make simple maps. A three-dimensional 
effect of terrain can be easily seen with the aid of a stereoscope. 

Oblique photographs have one factor in common; the camera is at a 
predetermined angle to the ground. Division into low and high oblique 
photographs refers only to the angle not to the altitude of the camera. 

The low oblique photograph can be defined as an angled camera shot that 
does not include the horizon. The angle provides a panoramic view of a larger 
area than does the vertical photograph. The photographs may be "restituted" 
by rephotography in the darkroom and made into verticals if their angles were 
low. Higher angles require special mapping techniques. Obliques may be used 
to bring maps up-to-date in small areas but the methods are tedious for larger 
areas . 

Principal use of this type of photograph is usually limited to the 
large-scale single photograph taken at low altitude, for public education, 
reports, demonstrations, and similar needs. 

The high oblique photograph includes the horizon. The perspective is 
natural or the way we normally see things. Reference to objects in the 
photograph is readily understandable because of its familiar angle of view. 
It is therefore, very good for displays, presentation of entire watersheds or 
other problems, especially with the public, where quick reference and 
recognition are desirable. The limitations of high oblique photography come 
in mountainous or hilly areas as slopes facing away from the camera are unseen 
or minimized. 

In summary , the Bureau confines most of its use to the vertical 
photograph because it requires minimum equipment to record the information in 
a usable form. This is related in turn to its relatively uniform scale, the 
map-like presentation of information, and its general availability and 

Obliques are more specialized. The specialization is advantageous in 
that the perspective may approach a normal viewpoint, thus interpretation is 
easy for the average person. 

All types demand infinitely greater integration into Bureau programs as 
they will perform services that cannot be achieved by other means. 

Fi 1ms 

The film used for most aerial photography is a dimensionally stable base 
film (e.g., Estar Base). Project requirements normally dictate whether black 
and white, color negative, color positive, or color infrared film should be 

Black and white photography of metric quality is most commonly used for 
photogrammetric operations such as precision topographic mapping and riparian 
surveys . 

Natural color (positive or negative film) is widely used by geologists 
and others to locate potential oil well drill sites as well as other energy 
sources. It is used extensively for resource inventories such as Forestry, 
Range, Wildlife, watershed, and others. 

Strip mining operations can be accurately monitored and trespass verified 
by proper utilization of this tool. There are many other applications for 
this kind of imagery in various disciplines. 

Color infrared (CIR) is extremely helpful in vegetative inventory work 
because of the ease in differentiating between species. Consequently it 
speeds up the interpretation process and increases accuracy of the interpreted 

Another application of CIR is in inventorying water sources (such as 
springs). When properly used the wet areas stand out clearly from surrounding 
vegetation. Most of the applications listed under natural color can also be 
accomplished with CIR. 

Fi Iters 

Filters are used for many purposes with both black-and-white and color 
films. The filters vary the contrast and tonal rendering of the subject in a 
photography, either to correct to the normal visual appearance or to 
accentuate special features. The photographic effect obtained with a 
particular filter depends on four main factors: (1) the filter's spectral 
absorption characteristics, (2) the spectral sensitivity of the sensitized 
material, (3) the color of the subject to be photographed, and (4) the 
spectral quality of the illuminant. 

Aerial views, when photographed without a filter, often appear veiled by 
atmospheric haze. This condition is caused by scattering of ultraviolet and 
blue radiation by water vapor and dust particles in the atmosphere. The high 
sensitivity of aerial films to these radiations compounds the effect. Since 
atmospheric haze scatters decreasing amounts of green and red light, and 
virtually no infrared radiation, haze filters are primarily ultraviolet and/or 
blue absorbers. Filters such as the KODAK WRATTEN Filters No. 2B and No. 2E 
absorb ultraviolet to reduce haze effect without affecting the monochromatic 
rendering of other colors. 

Greater haze reduction results from the use of yellow or red filters 
which absorb the shorter wavelengths. However, these filters also affect the 
monochromatic rendering of colors. The greatest penetration of atmospheric 
haze occurs when an infrared-sensitive material is used with a suitable filter 

Most aerial films require some type of filter in the camera system to 
eliminate the effects of atmospheric haze. Haze imparts an overall bluish 
cast to color aerial photographs and lowers the contrast in aerial 
black-and-white negatives. Filters that are appropriate to a particular film 
have been indicated in the data section for that film. In addition to the 
preceeding information fro"m Kodak publication M-29 , other technical data about 
filters may be obtained from Kodak publications M-5 , and B-3 as well as other 
sources . 


There are many types of photographic materials available that produce 
varying results, depending on the emphasis desired. Therefore, one should be 
aware of this in order to acquire the kind of product needed for a specific 
project. There are numerous kinds of paper that render different surface 
characteristics such as: glossy; high lustre; lustre; semi-matt; and matt. 
All of these have different degrees of smoothness or roughness that may 
require special pens or pencils to write on them. Various contrasts are 
available as well as different weights (thickness) of materials. The most 
commonly used for color photos is a resin-coated water resistant base paper, 
and probably a polycontrast paper would be the most common product used for 
black and white photo's. 

Also, there are stable base materials (similar to mylar) such as Kodak 

Duratrans Display Film, and Cronapaque by Dupont. Duratrans yields a color 

product and Cronapaque yields a black and white product that is scale stable. 
Of course, these will cost more than the paper products. 

A general rule is that the smoother the surface, the sharper the detail 
but the more difficult it is to write on. If you use an overlay it shouldn't 
matter . 

Photography Acquisition 

Considerat ions for planning a photo mission: 1) Intended use of the 
photography, 2) Who will fund the project, 3) Scope or size of project (how 
many square miles or lineal miles), 4) Season (start and completion dates) 
(What are you trying to emphasize), 5) Time of day to fly (especially for 
cliff areas), 6) Camera focal-length (3-1/2, 6, 8-1/4, or 12 inch), 7) Scale 
(depends on camera focal length and project needs), 8) Type of film (black. & 
white, color, or CIR) , 9) Type and number of prints (1 set contacts, or ?), 
10) Special requirements (film to wind in direction of flight, new film, etc.) 

BLM standard or special aerial photography specifications should be used 
to assure consistant quality imagery. Some of the specific requirements are 
discussed and illustrated in the following pages. 

Aerial photographs are taken on clear days when the sun is high enough to 
avoid distratingly heavy ground shadows. The project is flown at a specified 
flight height above mean terrain throughout the area. Each flight must 
parallel those adjoining for blocked areas that require multiple flights. 

Forward Overlap--usually 60% alon^ 
line of flight 

Line of flight 

Side overlap — usually 25% to 30% along 
adjacent flight strips. 

The ideal flight pattern is difficult to maintain. Sidewinds require 
flight corrections, the camera may be at an angle to the flight, then forward 
overlap and side overlap will grow inadequate with "crab." If the pilot fails 
to correct for the wind he will "drift" off the line of flight. Both "crab" 
and "drift" may require rephotography . See Figure 2 for an illustration of 
crab and drift. 

<■ •■■ s 























-*■< *'i *\ 

A. Desired pattern of flight 


r. . mi. .... 

B. Crab 

C. Drift 

Figure 2. Index flight paths. 

Titling and Indexing 

Photographs are individually titled. Each photograph is numbered 
consecutively as it was taken. Usually the date is included and the entire 
series may be as shown in the following example. 

Month /Day /Year 



Project ROLL/STRIP/Exposure 
Symbol No. 





CO-83 AC 





S 1 - 



N 1 - 



\ l " 



These photograph numbers indicate "full" coverage, 
Every exposure is necessary for stereoscopic work, 

These photographs are "alternates." Cost is cut one- 
half when prints are purchased. Does not give good 
coverage for stereoscopic work — but will suffice for 
simple planimetric maps. 




\ 1-5-li 




Indexes are an effective way to catalogue photo coverage for future use. 
An aerial photographic-index or spot (line) index may be used. These indexes 
identify the aerial photograph with the terrain. 

Aerial photo contractors usually will prepare a photo-index (photographic 
index) at additional contract cost, which varies with CIR, color, or black and 
white. Photo indexes are prepared by placing the photos in their relative 
positions with individual identity accented; a large photograph is then made 
of this group. If there are prominent features evident, the photo-index can 
be used for location of desired photos. A measure of control is present; 
townships can be superimposed on these photo-indexes with fair accuracy. 
Unfortunately an index of this sort usually is difficult to read and use, see 
Figure 2a. 

Spot indexes or line indexes are relatively inexpensive, especially if BLM 
provides the maps or matte mylars for their preparation to the contractor. 

The spot index is plotted on a map base showing flight lines and exposure 
stations. The land net on the map makes using this index very easy, see 
Figure 2b. 

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■ ■ • i)t ■ ■ , > f - 


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Park .. 

x in ^ 

Ui _« ► 

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at: ' UJ < 

cci _j> 

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A Pikes • ' * 
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Figure 2a. Typical photo-index. 

STRtM ;j§ ,r-STRTP2- 


Figure 2b. Typical spot (line) index. 

Displacements and Distortions 

Displacement due to relief is common within the photograph. The effect of 
elevation in photographic displacement is shown in Figure 3. The datum plane 
is identified for reference. Point "A" on a hill properly should be located 
on a true horizontal distance from the center of the photograph at "A-*- , " but 
is displaced outward in the photograph to "A^." Point "B" is below the 
datum plane. Its correct location on the datum plane is displaced toward the 
photograph center, in this case to "B^." A matter for emphasis is that in 
both situations the displacement of a point inward or outward is on a straight 
line from the center of the photograph. 

Displacement of a point then is caused by the effect of relief. No matter 
to what degree points are displaced they move inward or outward on a line 
radiating from the center. A compass bearing from the center to the object 
point will not change with displacement. A hilltop, in the several 
photographs surrounding it, is displaced outward on a line from each center. 
Assume this center of each photograph is located correctly in its relative 
ground position. Every line bearing on the hilltop from each center is on a 
true bearing. The point where the lines intersect represents the true 
horizontal location of the hilltop. 

Intersecting rays from the centers of overlapping photographs are plotted 
to locate points in true horizontal position and thereby eliminate 
displacement due to relief. This is the basis of the "radial-line" and 
"slotted-templet" method of map preparation. We can adapt this technique to 
locate single objects, such as a new radar site on a hill. 

Remember : 

Points above datum plane are displaced outward from the center. 

Points below the datura plane are displaced toward the center. 

Displacement is radial relative to the center of the photograph. 


iZ . 

A to A = Distance of displacement away from center of photograph 

2 _ 

B to B = Distance of displacement towards center of photograph 

»i - 

A and B = Corrected positions 


A and B = Displaced positions on photograph. 

Figure 3. Displacements and distortions. 





Figure 4. Displacement due to relief. 


Figure 4 illustrates a square section with rugged terrain. The centers of 
the section and photograph coincide. The Sw and SE corners lie on the datum 
plane, whereas, NW is above and NE is below it. NW will displace outward, 
radially, from the center and NE will displace inward, radially, toward the 
center because it is located in a deep canyon. The square section is 
illustrated with solid lines; dotted lines show the distorted view of the 
section as it is depicted in the photograph. 

Distort ion in the photograph may be caused by several things, e.g., photolab 
processing, film or paper instability, camera lens, tilt or tip of the camera 
at the instant of exposure, etc. 

Photolab processing accounts for some minor distortion in photographs. 
The wet paper print may stretch in one direction and shrink, in another 
dimension upon drying. Such errors are lessened by use of resin coated 
paper. Single-weight paper is used in mosaics where stretching sometimes is 
done purposely for a match of detail. 

Distortion from the camera lens may be present. If the photographs are 
enlarged, the enlarging may introduce further errors. This difficulty can be 
minimized by confining your work, to the center of the photograph (or neat 
model). Use more photographs if necessary. Modern "distortion-free" camera 
lens have minimized this problem. 

Tilt and tip causes the photo to have a distorted scale similar to that of 
a low oblique photograph but is uncontrolled. Tilt is divided, sometimes, 
into two parts--tips and tilt--for computing corrections. Tip in a photograph 
is caused by the airplane climbing or diving at an angle to the ground. At 
right angles to tip is "tilt" as the airplane banks in flight. Bureau 
specifications state that tilt shall not average more than 2° in any flight 
line nor more than 1° for the entire project. Relative tilt between any two 
successive exposures exceeding 6° may be rejected. 

Photographs contain invaluable information but also have errors 
(displacements and distortions) which must be recognized and, if necessary, 
corrected. You can correct many of these errors as you gain experience but 
for now to minimize the problems: 

From photolab processing; order resin coated or other stable base 
materials . 

From camera lens; use the center portion of photographs. 

From tip and tilt; require photography of less than 3 percent error. 


Scale Considerations 

The scale of an aerial photograph is a vital piece of knowledge for your 
work. It is the ratio of a distance on the photograph to its actual distance 
on the ground. This ratio of photo distance: ground distance (PD:GD) is the 
scale. It is expressed usually in three ways: 

A. Descriptive scale 

The descriptive scale is in common use. It is a statement of ratio in 
familiar terms, such as "1/2 inch = 1 mile." It is awkward, however, because 
two different units of distances are used, one for the ground and one for the 
photograph. Inches on the photograph are related to miles on the ground. 

B. Graphic scale 

The graphic scale permits direct measurement on the photograph in a 
convenient unit (miles, yards, feet). There is no calculation in its use; 
hence it is rapid and easily used. 

C. Representative fraction scale or RF 

The representative fractions (RF) is the ratio of photograph distance 
to ground distance with both distances expressed in the same unit of measure. 

photo distance PD 1 GD 

Scale = or — or PD:GD = corresponds to 1: — 

ground distance GD GD/PD PD 

This is the basic statement of scale for all map and photograph work. It is 
best suited for calculations. Example of an RF is 1:63,360, which is the same 
as stating "1 inch = 1 mile" (63,360 inches). Thus the RF is really the 
familiar descriptive scale with the confusion of two units of measure removed. 

The scale of an aerial photograph is determined by two factors at the moment 
it is taken (Figure 5). 

1. Height of airplane above the ground (not the altitude of the plane). 

2. Focal length of the camera lens taking the photograph. 

Problem: Scale is calculated by the ratio :H (focal length : Height 
above-ground. Both in same unit of measure). Find the scale when airplane 
altitude = 10,000', focal length of lens is 12", ground elevation is 3,000 

Solution: Subtract 3,000' from altitude = 7,000', or H. f = 12 inches or 1 
foot. Scale = f:H, or 1:7,000. 

1 4 

The scale of photographs is usually given as a representative fraction (RF) 
and written on one line (1:5, 000). If a vertical photograph contains the 
altitude and focal length of the camera, the scale is obtained as shown below. 

Scale = RF=-j^- = f:H 

Corresponds to I: -7- 

f = Focal length 
in feet 

Note: f and H must be in the 
same unit of measure. 


Figure 5. Determination of photo scale from lens focal length and altitude 


Scale = 



Scale on mesa top 

9,000 (ABOVE GROUND) " 18,000 
I Mile = 3.5" 

— Corresponds to I:— 

Scale on valley floor 




10,000 20,000 
I Mile = 3.2" 

Figure 6. Effect of elevation on scale 


Scale varies with the height of the camera above the surface of the datum 
plane or mean terrain elevation. It follows then that the scale varies within 
each photograph with every elevation change. Figure 6 (Effect of elevation on 
scale) shows this problem. However, the effect of terrain height on the photo 
scale is not confined to such violent contrasts. A tall building will have a 
different scale at its base from that of its roof. 

You may be classifying a perfectly square section lying on a gently 
sloping side hill with only a small grade (a rise of 300 feet in 1 mile). The 
section will be deformed on a photograph. At a scale of 1:21,000, the low 
side of the section will measure 3.0 inches; the higher side 3.1 inches--a 3% 
error . 


Scale Determinations 

A quad map is good for calculating the scale of a photograph. The same 
points must be recognizable on both map and photograph. Each point should be 
located as far distant from the other as possible and be within your project 

The calculation of Figure 7 is typical for one pair of points. Here "A" 
and "B" were recognizable on both map and aerial photograph. First determine 
the actual ground distance between "A" and "B." The distance between them on 
the map is 1.2 inches. Map scale is 1:50,000. Therefore, ground distance is 
1.2" x 50,000 = 60,000 inches from "A" to "B." Now to determine the 
photograph scale as the ratio of photo distance to ground distance (PD:GD). 
PD is 2.5" on the photograph (by measurement) and GD is 60,000" (calculated 
from the map). The ratio, therefore, is 2. 5" : 60,000. " Divide both parts of 
the ratio by 2.5 which gives the answer: 2.5 f 2.5 = 1 

or 1:24,000 

60,000 v 2.5" = 24,000 
which is the photo scale. 

The scale of a photograph is easily determined in the field . A pair of 
points are located on the photograph along roads or routes that can be 
measured easily. Example: Assume the distance between two points on a road 
is 1.7 miles and the distance measured on the photograph is 2.3 inches. Find 
the photograph scale. 

Converting miles to inches gives all measurements in a common unit (1.7 x 
5280 x 12" = 107,712"). The scale PD:GD 2. 3 ' ; 107 , 712" = 1:46,831, or, in 
round numbers, 1:47,000. 

By comparison , the photograph will sometimes reveal square sections. A 
quick measurement can give the inches per section on the photograph, perhaps 
3" = 1 mile (3"=63 , 360") , which can be resolved by inspection into the nominal 
scale of 1:21,000. 

'hoto -scale unknown 


Map -scale 1:50,000 

Distance AB = 2.5" on photo 

Distance AB = 1. 2" on map 


Determine scale of photograph. 

Scale s ratio of photo distance to 
ground distance. 


1.2" x 50,000 = 60,000" ground 
distance between A and B . 

Photo scale 

photo distance 
ground distance 


Photo Scale - 

2.5" + 2.5" = 




60,000" -r 2.5" = 24,000 

or 1 : 24,000 

Figure 7. Determination of photo scale 


Acreage Determinations 

Some typical uses of photo scale are for: locating legal subdivisions 
(sectionizing) ; determining the size of objects on the ground such as dams, 
buildings, recreation areas, etc.; and calculating approximate acreage 
directly from the photographs. 

Many field offices have access to digital planimeters that calculate the 
acreage when a photo scale is entered. Another technique that can be used in 
the field is the Dot Grid method. Examples for determining the value of a dot 
and acreage calculation are as follows. 

Dot value for Dot Grids example: Photo scale 1:24,000 

Step one: Convert photo scale to feet per inch, 

answer; 24,000 *■ 12 (inches per foot) = 2,000' or 1 inch of photo coverage 
is equal to 2,000' on the ground. 

Step two: Determine how many square feet are in 1 square inch at a scale of 
1:24,000 or 1" = 2,000' . 

answer; 2,000' x 2,000' = 4,000,000 square feet in 1 square inch of photo 
coverage at 1:24,000 scale. 

Step three: Determine how many acres are in 1 square inch at 1:24,000 scale. 

answer; 4,000,000 sq. ft. *■ 43,560 (number of sq.ft. per acre) = 91.8 
acres per square inch of photo coverage, at 1:24,000 scale. 

Step four: Determine the value of each dot at a scale of 1:24,000 

answer; 91.8(acres) *■ 100 (no. of dots per square inch) = .918 or .92 
acres per dot. 

Note: The reason for converting scale 1:24,000 to feet per inch is to get the 
formula; feet times feet = square feet (4,000,000 in this example) and convert 
that to acres. It also accommodates pocket calculator limitations. 

Use of Dot Grid — randomly lay the dot grid over the area to be measured 
(polygon, rectangle, square, etc.) and count each dot, (count the dots that 
fall on the boundary line(s) as one half dot) and multiply the total by .92 to 
determine the number of acres in an area at 1:24,000. This should be done two 
or three times for each area and average the results. 


Acreage Determinations Continued 

The following examples show how acreage can be calculated on square, 
rectangular, or triangular areas once the photo scale has been determined. 

For both examples assume a photo scale of 1:24,000 or 1" on the photo is equal 
to 2000 feet on the ground. 

Example No. 1 - You have an area on the photo that is 1.1 inch wide and 3.4 
inches long. 


Multiply 1.1" x 2,000' = 2,200' wide 

Multiply 3.4" x 2,000* = 6,800' long 

Width (2,200') x length (6,800') - 14,960,000 sq. ft. in the area 

14,960,000 * 43,560 (no. sq. ft. in 1 acre) = 343.4 acres 

(Note: This same formula works for a square area.) 

Example No. 2 : You have a triangular area that is 1.1" high and 3.4" long as 
follows : 


Multiply 1.1" x 2,000' = 2,200' (height) 
Multiply 3.4" x 2,000' = 6,800' (length) 

2,200 x 6,800 = 7,480,000 sq. ft. in the triangle 

7,480,000 4- 43,560 (no. sq. ft. in 1 acre) = 171.72 acres in triangle 

Figure 8. Acreage determinations. 


Image Characteristics 

During interpretation of aerial photographs occasionally the terrain 
features will "reverse" themselves; that is, the drainages appear as the 
highest points. Turn the photographs around; this will force the features 
into their proper relationship. 

Direction is sometimes a very important aid to the interpreter. In 
northern latitudes the shadows in the photograph fall toward the northwest if 
taken in the morning and northeast for those taken in the afternoon. North 
may be determined with greater certainty by comparing the photo with a map of 
the area or orienting by compass. Resource photography flights are usually 
flown in cardinal directions — east-west or north-south — with 
photo-identification uniformly presented at one end of the photograph — the 
west or north side. (SPECIAL FLIGHTS, e.g., stream photography or other 
special projects may not be flown in cardinal directions, therefore, the 
titled information may not indicate direction). 

Some of the photo image characteristics that assist in the identification of 
objects are illustrated in Figures 9 and 10 and are listed and elaborated on 
in the following pages. 

Shape - Shape may identify. 

Size - Comparative sizes may identify. 

Shadow - Shadow is often a clue by characteristic shape. 

Tone/Color - Photographic tone or color is the characteristic colors 
and/or gray shades of scenes and objects. 

Texture - Texture in aerial photographs is created by the frequency of 
tonal or color changes. 

Pattern - Pattern is a more or less orderly arrangement of manmade objects 
or natural elements. 

Relation to surrounding objects - This subhead might just as well be 
labeled "Deduction." Actually all interpretation hinges here to some degree, 
consciously or unconsciously. 


Shape . The shapes of objects seen in vertical view are sometimes surprisingly 
difficult to interpret. The plan or top view of an object is so different 
from the familiar profile or oblique view that inexperienced interpreters have 
failed to recognize the image of the building in which they were working. The 
ability to understand and make use of the plan view has to be acquired like 
another language. It then becomes a powerful tool, for the plan view of 
objects is an important and sometimes conclusive indication of their 
structure, composition, and function. To the interpreter who is experienced 
in industrial studies, the vertical view of a factory tells more about its 
function than a stroll past its front door. The vertical view of a forest may 
reveal its economic and recreational value. The vertical view of a landform 
may show spectacular effects of tectronic and gradational processes. To the 
motorist, a cloverleaf road intersection is an incomprehensible maze through 
which one must find his/her way by faith and strict attention to signs; to the 
aerial observer, the intersection is perfectly clear in form and function. 

Much of the training of the photo interpreter is aimed at the 
reorientation of perceptions; so that he/she can easily recognize objects seen 
from above. This reorientation is greatly aided by the impression of depth in 
stereoscopic pairs. 

Size . The size of an object is one of the most useful clues to its identity; 
by measuring an unknown object on an aerial photograph, the interpreter can 
eliminate from consideration whole groups of possible identifications. An 
irrigation ditch and an anti-tank ditch, for example, are very much alike 
except in size and a simple measurement may suffice to make the 
identification. It is always advisable, when faced with an unknown object, to 
measure it. When working with photography on variable scale, the interpreter 
should make frequent measurements of the objects of interest. 

The interpreter can avoid errors in identification by paying attention to 
the size of objects. Misidentif ications are possible even though the 
interpreter may have carefully considered such clues as shapes, shadow, tone, 
texture, and pattern. 

Shadows . Shadows are familiar phenomena, and in ordinary life we often judge 
the size and shape of objects or persons by observing the shadows they cast. 
The shadows present in aerial photographs sometimes help the interpreter by 
providing profile representations of objects of interest. Shadows are 
particularly helpful if the objects are very small or lack tonal contrast with 
their surroundings. Under these conditions the sharp tonal gradients of the 
shadows may enable the interpreter to identify objects which themselves are 
just at the threshold of recognition. 

2 3 

a. Shape 

b. Relative size and pattern 






d. Shadow 

(C) (D) (E) 

e. Relation to surroundings 






Figure 9. Identification of objects. 


Useful for large objects and large scale photography. 

Shadows identify objects as a wind 
break of Lombardy Poplar. 

Shadow identifies object as a water 
storage tank . 

Watch out for cloud shadows ! 
Cloud shadows give a dark tone that 
bears no relationship to the terrain. 
However, in flat level country, these 
shadows may puzzle photo users. 

Sometimes there are shadows without distinguishable objects. 


. fjnwiLT.^.VJ 

■ m. m .wf . jA"3> Vfi ^ CT=l 


Figure 10. Shadows. 


Tone . Tone of a photograph is a relative blackness or whiteness and is the 
result of the amount of light reflected by an object. Tone is fundamental to 
interpretation of black-and-white photography and, used with other recognition 
elements, is a primary element for feature identification and interpretation. 
The tones of photographic images are influenced by many factors, and the tones 
of familiar objects often fail to correspond to our perceptions of those 
objects in nature. A body of waste may appear in tones ranging from white to 
black, depending on the angle of Sun and the number of wave surfaces 
reflecting light to the camera lens. A black asphalt road may appear very 
light in tone because of its smooth surface. A trail may appear white in dry 
weather and dark after a rain. A smooth, rounded, metal object of 
indeterminate color, like a tank truck, may reflect so much light that the 
tonal range of the film cannot record it and details are lost. Figure 9 
illustrates degrees of reflection, under "tone." 

When the photo interpreter understands the factors which govern 
photographic tone, he/she regards the tones of objects of interest as major 
clues to their identity or composition. The soil scientist uses tonal 
variations to classify soils; the forester, to distinguish hardwood from 
coniferous trees; the geologist, to map lithology and structure or prospect 
for minerals. In single photographs, where the shapes of objects must be 
inferred from monocular clues, or in stereoscopic pairs if the objects of 
interest have little or no visible height, tone is particularly important. 

Color . A feature has a color when it reflects particular wavelengths of 
light. For example, vegetation appears green because the properties of the 
plant preferentially reflect a larger percent of green light compared to blue 
and red. Hue (color), value (saturation), and chroma (brightness) are the 
three variables that constitute color. The human eye can distinguish many 
more hues of color than it can tones of gray. Thus color permits recognition 
and interpretation of a greater amount of detail. In the interpretation of 
rocks, soils, and plants where there may be an abundance of features whose 
natural colors are important, some workers have found that color photography 
pays for itself in abundance and accuracy of information. False-color 
infrared film, developed during World War II, has been found useful for 
special studies of plant conditions, vegetation distribution, soil-moisture 
conditions and drainage delineation. 


Texture . Relations of texture on tone are illustrated in Figure 11. Heavy 
grazing removes many of the tiny traps of sunlight — grass blades. Reflection 
is high with the grasses removed so the tones grow light. Ungrazed grasses 
give darker tones. As grazing is seldom the same on both sides of the fence, 
fences may be located by this difference in texture and tone. Sometimes stock 
will travel the fence line, removing all vegetation and making the surface 
very smooth. The result is a white line or trail paralleling the fence. 

Taller grasses trap more light and reflect little toward the camera. 
Meadows with greater densities of grass will be darker accordingly than 
surrounding grassland. Shrubs and trees reflect little light and usually are 
very dark. The tone will vary with individual species. 

In general, the smoother the surface or texture, the lighter its tone in 
the photograph. Dark tones come from rough textures — tall grasses and 
shrubs. Color is often secondary in its effect on tone when it is combined 
with texture. Check most carefully in puzzling areas where location is 
important (as in pricking a section corner on a photograph) to assure that the 
effects of texture on tone is evaluated properly. 

Texture in aerial photographs is created by 
color change in groups of objects which are too 
individuals. It follows that the size of object 
varies with the scale of photography. In large- 
be seen as individuals; their leaves or needles 
separately, but contribute to the texture of the 
of smaller scale, the crowns contribute to the t 
trees. Within a given range of scales, the text 
(e.g., a timber stand of a certain species compo 
enough to serve as a reliable clue to the identi 

the frequency of tonal or 
small to be discerned as 

required to produce texture 
scale photographs, trees can 
cannot be discerned 

tree crowns. In photographs 
exture of the whole stand of 
ure of a group of objects 
sition) may be distinctive 
ty of the objects. 


Texture is an important interpretative factor in using images acquired 
from orbital altitudes. For example, the relative erosional dissection of 
area may be inferred only by its texture because individual drainageways 
cannot be delineated. Drainage pattern is an important indicator of the type 
of surficial materials and bedrock. In other cases, features may be similar 
in color but may exhibit considerable difference in texture (e.g., volcanic 
fields versus desert pavement). 

Fence shows on photograph 
as a deviding line between 
the two gray tones . ->. 

Rougher texture, tall grasses 
with great density 
Darker gray tone 

Smooth surface 
Heavy grazing 
High reflection 
Light gray tone 

Rough texture 
Ungrazed grasses 
Medium reflection 
Dark gray tone 

Coarse texture of willows 
Little reflection 
Black tone 

Figure II. Effect of surface and texture on black and white photographs 



3-^^-1*^% " 

Open level range with lighter meandering 
tones converging could indicate a windmil 
Trails and smooth textures from closer 
grazing gives pattern. 

Tanks and surrounding retention dikes 
indicate oil storage. 


Photographic view 

Ground view 

Very sandy soils or sands migrating over clay sub soil . 

Figure 12. Patterns. 

2 8 

Patterns . Students of earth science have always laid great stress on the 
pattern or spatial arrangement of objects as an important clue to their origin 
or function. Geographers and anthropologists study settlement patterns and 
their distribution in order to understand the effects of diffusion and 
migration in cultural history. Outcrop patterns provide clues to geologic 
structure, lithology, and soil texture. The varying relations between 
organisms and their environment produce characteristic patterns of plant 

Regional patterns which formerly could be studied only through laborious 
ground observation are instantly and clearly visible in aerial and space 
photographs. Aerial photographs capture many small but significant patterns 
which might be overlooked or misinterpreted by the ground observer (e.g., 
fracture traces and "halos"). Innumerable variations in classic patterns can 
be seen and exploited by means of image interpretation. 

Some patterns are primarily cultural and others are primarily natural. 
There are, however, few parts of the world which have not been affected by 
man, and most of the patterns visible in imagery result from the interaction 
of natural and cultural factors. Even the photogeologist must sometimes take 
the activities of man into account. 

Cultural features are conspicuous in aerial photographs because they 
consist of straight lines or other regular configurations. Most of man's 
activities leave scars on the earth which persist for a long time even after 
the activities have ceased. Patterns of settlement, mining, and agriculture 
may be visible from the air after thousands of years, directly or through 
altered patterns of vegetation and erosion, and photo interpretation has 
become an important technique of archaeology. 

Patterns formed by agricultural practices, fracture alignments, drainage 
networks, and vegetation are important factors in the interpretation of an 
image. As mentioned above, intricate patterns may be reflected as a textural 
difference in space imagery. In many instances, regional patterns associated 
with other image elements provide clues to finer-grained patterns. 

Patterns indicate relationships through repetition or arrangement of 
objects or form. The relationship may be the typical patterns imposed on a 
streambeo by geology, for example: 

Dendritic (Homogeneous 
or flat-lying 
sedimentary rock) 

Trellis (tilted or 
folded sedimentary 

Rectangular (well- 
developed joints and 

The relationship may be typical patterns of land use, the symmetry of oil 
tanks surrounded by circling dikes, and soil conditions (Figure 12). 


Parallel ridges might be a geological structure, but road net leading to 
railway siding indicates strip mining. 

Diversion dam with ditch might be 
irrigation canal, but ditch is leading 
to tailings which indicates a flume 
used in a mining operation . 

Figure 13. Relationship to surrounding objects. 


Relationship . A link must be established in the chain of identification on a 
photograph. This link is the summation of the relationships of objects on the 
photograph. The other guides or keys to identification and interpretation 
sometimes are not enough. 

A railroad may be easily identified (Figure 13). It ends in a field 
abruptly. In time of war, it could well be a railroad gun or a guided missile 
site, especially with a spur parallel to it. However, a sugarcane field might 
be ready for harvesting and these are the temporary tracks. The 
identification requires a relationship not only to the ground but to many 
factors implicit to the situation on the ground but perhaps incapable of being 
expressed in a photograph. This is where you, your background, and experience 
lead to a correct interpretation of the photograph. 

A broad knowledge of cultural, economic and social factors means much in 
this phase of interpretation. A single example is given in Figure 9. A road 
going to a stream and continuing on the opposite bank, plus the lack of a 
bridge, is an indication of a ford or a ferry. The parking lot for cars 
awaiting for the return trip suggest a ferry. 

Knowledge of local customs that influence structural design may help. 
Churches in Europe are oriented toward the east. The direction of the church 
is determined by the cross-like character of the building — east is at the 
altar. Customary driving on the roads (left or right) affects construction of 
cloverleaf intersections — an indication of the unconscious impact of customs 
of a country on design. 

The obvious relationship of railroads to sidings, the abrupt curves of 
secondary roads, the broad sweep of a super-highway are more than just that. 
They represent our evaluation of the everyday happenings in our like as we see 
them, readily recognize the conditions, and relate them to the photograph. If 
we carry this relationship further in careful evaluation of conditions, not 
only on manmade features but geological and soil features as well, as a cause 
and effect, we are entering into the highly skilled field of photo 
interpretation where many of the Bureau employees rightfully belong. 


Techniques and Procedures 

Certain image-interpretation techniques, when properly applied, can 
improve the quality and quantity of useful information extracted from 
imagery. Among the techniques being refined are methods for using (1) 
methodical procedures, (2) efficient search techniques, (3) knowledge of 
factors-governing image formation, (4) the background and training of the 
interpreter, (5) the concept of "convergence of evidence," (6) the "conference 
system," (7) information available in analogous areas, (8) reference 
materials, (9) simple and sophisticated equipment, and (10) acquiring field 

The image interpreter must have an understanding of: (1) how the image 
was formed, (2) what the image elements represent, and (3) the basic earth 
processes and phenomena that are present on the image. The amount of 
information that an interpreter can extract from an image is proportional to 
his/her experience, skill, and interest. 

Efficiency in the analysis of imagery is dependent upon a number of 
factors. A systematic approach to the problem under study is perhaps the most 
important consideration. In general, a systematic analysis involves the 
convergence of empirical evidence drawn from the imagery. The following 
procedures (steps) illustrate this method: 

3. Collateral information 

a. Reports 

b. Maps 

c. Field data 

4. Summary of interpretation results 

5. Field check in key areas 

1. Regional analysis 

a. Geographic aspects 

b. Physiographic aspects 

c. Geology 

d. Climate 

2. Local analysis 

a. Land forms 

b. Drainage patterns 

c. Erosion . 

d. Image patterns 

e. Vegetation 

f. Spectral features 

The approach works and results in a savings of time. Additional benefits 
and information are also obtained by using a multidisciplinary/ 
interdisciplinary team approach. 


Photo Preparation 

The Effective Area is that central portion of a vertical photograph delimited 
by the bisectors of the overlaps with adjacent photographs. On a vertical 
photograph, all images within the effective area have less displacement than 
their conjugate images on adjacent photographs. 

Effective area boundaries are important in photo interpretation for two 
reasons: (1) on photography of mountainous country, they define the smallest 
usable area of least displacement on each photograph, and (2) they provide 
interpretation "match lines" between photo pairs which avoid duplication or 
gaps in delineation between photographs — both within, and between, lines. 

When determining effective area boundaries proceed as explained and 
illustrated in the following, (Substitute your own numbers instead of the 
examples cited). 

In-Line Effective Area Boundaries . 

o Place photo 143 over photo 144 in overlap position with flight line segments 
coinciding (Step 1, Figure 14). Don't tape down. 

o Visually locate and pencil mark, the endlap midpoint on the west side of 
photo 144. Now slide 143 across 144 perpendicular to the line of flight until 
the format margin on the east side of 144 is showing. Mark, the endlap 
midpoint on the east side of 144 (Steps 1 and 2, Figure 14). 

o Connect these overlap midpoints on 144 with a ruler and colored lead pencil. 

o Stereoscopical ly , or visually without a stereoscope, transfer 4-5 points 
along this mid-overlap line to photo 143. Do not pinprick! Accomplish 
quickly with pencil and locate the points at extremes of relief (ridge tops, 
valley bottoms, etc.). 

o Connect the transferred pencil points on 143 with colored pencil (Figure 
15). If there is considerable variation in relief, the transferred boundary 
will be crooked — and it is! Viewed stereoscopical ly , if properly 
accomplished, the two lines will merge and "flow" over the surface of the 

o Locate the in-line effective area boundaries between stereo pairs 143-142, 
229-230, and 230-231. 

o On the end photos in the flight lines (142, 144, 229, 231), rule straight 
pencil lines through the Principal Points (PPs) (Photo Centers), across the 
line of flight, to within about 1" of the east and west fiducial marks on each 
of these photos as in Step 1, Figure 16. 

Between-Line Effective Area Boundaries . 

o Place the two lines of photography side-by-side in front of you with the 
titling away from you (Step 1, Figure 16). 

o Overlap 229 over 144 on the basis of matching ground features to determine 
the sidelap zone. 

3 3 




r- — 



a 1 

T3 • 
c 1 

UJ | 


1 1 


v — Pencil mai 



— __ _ 

^•Pencil mai 

- — Slide 144 in this 
direction to expose 
edge of photo 143 


• i 


Endlap zone 



Figure 14 

Location of endlap 

Figure 15. Transfer of endlap bisector 
on photo 144 to 143 results in 
a crooked line on 143 due to 
relief variations. 





In-line effective 
area boundaries 


Sidelap midpoint 


":■.: •■••'vi 






effective area 


Sidelap zone 




— •* 

■ — c 







In the absence of an adjoining line of 
photos, this boundary is drawn about 
1.25 inches from the photo edge. 


Figure 16. Location of the in-line 

effective area boundaries. 


o Visually locate the sidelap midpoints and rule in as a straight line on 
229. Do not extend the line beyond the endlap effective area boundaries (Step 
2, Figure 16). 

o Visually transfer this line to its conjugate position on photo 144. When 
there are extremes in relief in the sidelap zone, this line will be irregular 
— reflecting variations in topographic displacement (Step 3, Figure 16). 

o Repeat procedure between 143 and 230, 142, and 231. 

o Since there are no adjoining photographs on the east of 229-231, or on the 
west of 142-144, put in the final side of each effective area at about 15% of 
the width of the photo from the edge (circa 1.25 inches) - as illustrated in 
Step 4, Figure 16, for photos 143 and 230. 

o Please note that, where transferred boundaries are indicated in the above 
diagrams, the degree of irregularity has been purposely exaggerated. 

o It is not unusual for the effective area boundaries to sometimes be located 
on alternate photos - particularly in areas with little, or no, steep 
topography. In such cases, the effective area boundaries are determined in 
precisely the manner described above - but between every-other photo. 

Overlay Preparation : 

Clear acetate overlays (9 in. x 9 in.) should be placed on each frame to be 
interpreted. A fine point, permanent marking pen should be used. Delineate a 
line over each fiducial mark observed on the photo. Trace the photo number 
(e.g., 143) precisely over the lettering on the print. Trace the match lines 
precisely from the print. The interpretation overlay is now registered to 
either a particular transparency or its paper print. It can be removed from 
over the photo and replaced without fear of misregistration. Figure 17 shows 
examples of properly prepared overlays. 



Interpret this area 
between match lines 
for each photo . 

Riparian area 

Figure 17 

Figure 18 is an overlay representing typical classifications of vegetation 

within a specific area. Symbols O/P is oak and pine with oak being dominant 

and pine sub-dominant. D/P is Douglas fir and pine, O/G is oak and grass, etc. 

Figure 19 is a typical drainage overlay. 




Figure 18. Typical vegetation overlay. 

3 7 



Figure 19. Typical drainage overlay. 




I wish to express my sincere appreciation to the following individuals for the 
use of information originally prepared by them for special purposes and/or 
presentat ions : 

Donald T. Lauer (Chief, Technique Development and Applications Branch, EROS 
Data Center, U.S. Geological Survey), Dr. Merle Meyer (University of 
Minnesota-Retired), and Richard Burr (BLM-Retired) . Special recognition is 
appropriate for Herman E. Weiss (BLM-Visual Information Specialist) for his 
invaluable assistance with the development of illustrations and with 
requirements for printing this publication. 





BLM/YA/PT -88/001+1268 

4. Title and Subtitle 

Interpretation of Aerial Photographs 

7. Author(s) 

Wallace A. Crisco 

9. Performing Organization Name and Address 

U.S. Department of the Interior 

Bureau of Land Management Service Center 

P.O. Box 25047 

Denver, CO 80225-0047 

3. Recipient's Accession No. 

5. Report Date 

August 1988 

8. Performing Organization Rept. No. 


10. Project/Task/Work Unit No. 

11. Contract(C) or Grant(G) No. 



12. Sponsoring Organization Name and Address 

U.S. Department of the Interior 

Bureau of Land Management Service Center 

P.O. Box 25047 

Denver. CO 80225-0047 

13. Type of Report & Period Covered 


15. Supplementary Notes 

16. Abstract (Limit: 200 words) 

This publication provides information on basic interpretation of aerial photographs, 
Included are discussions on types of photography, films, and filters. Photo 
interpretation and usage are also covered, with discussions of scale and acreage 
determinations, image characteristics, and photo preparation. 

17. Document Analysis a. Descriptors 

Aerial photographs 
Aerial photography 
Photography 1405 
Photo interpretation 

b. Identifiers/Open-Ended Terms 



c. COSATI field/Group 

18. Availability Statement 

Release Unlimited 

19. Security Class (This Report) 


20. Security Class (This Page) 


21. No. of Pages 


22. Price 

(See ANSI-Z39.18) 

See Instructions on Reverse 

OPTIONAL FORM 272 (4-77) 
(Formerly NTIS-35) 
Department of Commerce 

ii U.S. GOVERNMENT PRINTING OFFICE: 1988—573-003/80,819 REGION NO 8 

BLM Library 
D-553A, Building 50 
Denver Federal C«ftt«* 
P.O. Bos 25047 i 
Denver, CO 80235-004?